That error is also generated even if the method exists but you call it with the wrong number of arguments or the wrong types for the arguments (this is because D-Bus supports Polimorphism).

You might want to read the following thread:
https://bbs.archlinux.org/viewtopic.php?id=114632

As far as I know Sailfish uses the standard "Freedesktop" notification interface, which means it is the same that is used on Gnome, KDE, and other GNU/Linux desktop environments (finally!). This means that any examples designed for those environments should work in your Jolla device, too.
In desktop environments the "notify-send" program can be used to send the appropiate D-Bus calls, too.